Polymer compositions useful for molding applications, specifically the manufacture of caps and closures for bottles are well known. Screw closures for example, are typically made from polypropylene (PP) in order to achieve the necessary cap strength, however, an inner liner composed of a soft polymer is required to provide necessary seal properties. The soft inner liner can be made from ethylene/vinyl acetate (EVA), polyvinyl chloride (PVC), butyl rubber or other suitable material. The two-part cap is costly, and single part constructions are preferred to reduce cost.
Accordingly, one-piece closures, such as screw caps have more recently been made from polyethylene resins. The use of high density resin is required if the closures are to have sufficient stiffness, while broader molecular weight distributions are desirable to impart good flow properties and to improve environmental stress crack resistance (ESCR).
Polyethylene blends produced with conventional Ziegler-Natta or Phillips type catalysts systems can be made having suitably high density and ESCR properties, see for example, WO 00/71615 and U.S. Pat. No. 5,981,664. However, the use of conventional catalyst systems typically produces significant amounts of low molecular weight polymer chains having high comonomer contents, which results in resins having non-ideal organoleptic properties.
Examples of high density multimodal polyethylene blends made using conventional catalyst systems for the manufacture of caps or closures are taught in U.S. Pat. Nos 2005/0004315A1; 2005/0267249A1; as well as WO 2006/048253, WO 2006/048254, WO 2007/060007; and EP 2,017,302A1. Further high density, multimodal polyethylene blends made by employing conventional Ziegler-Natta catalysts are disclosed in U.S. Pat. Nos. 2009/0062463A1; 2009/0198018; 2009/0203848 and in WO 2007/130515, WO 2008/136849 and WO 2010/088265.
In contrast to traditional catalysts, the use of so called single site catalysts (such as “metallocene” and “constrained geometry” catalysts) provides resin having lower catalyst residues and improved organoleptic properties as taught by U.S. Pat. No. 6,806,338. The disclosed resins are suitable for use in molded articles. Further resins comprising metallocene catalyzed components and which are useful for molding applications are described in U.S. Pat. Nos. 7,022,770; 7,307,126; 7,396,878 and 7,396,881 and 7,700,708.
U.S. Pat. Appl. No. 2011/0165357A1 discloses a blend of metallocene catalyzed resins which is suitable for use in pressure resistant pipe applications.
U.S. Pat. Appl. No. 2006/0241256A1 teaches blends formulated from polyethylenes made using a hafnocene catalyst in the slurry phase.
A bimodal resin having a relatively narrow molecular weight distribution and long chain branching is described in U.S. Pat. No. 7,868,106. The resin is made using a bis-indenyl type metallocene catalyst in a dual slurry loop polymerization process and can be used to manufacture caps and closures.
U.S. Pat. No. 6,642,313 discloses multimodal polyethylene resins which are suitable for use in the manufacture of pipes. A dual reactor solution process is used to prepare the resins in the presence of a phosphinimine catalyst.
Narrow molecular weight polyethylene blends comprising a metallocene produced polyethylene component and a Zielger-Natta or metallocene produced polyethylene component are reported in U.S. Pat. No. 7,250,474. The blends can be used in blow molding and injection molding applications such as for example, milk bottles and bottle caps respectively.
In U.S. Pat. Appl. No. 2010/0261848A1 we disclosed a resin composition having a good balance of toughness, ESCR, processability, and organoleptic properties for use in the manufacture of caps and closures. The resins were made using a single site catalyst system in a dual reactor solution process, to provide bimodal polyethylene compositions in which comonomer was present in both a high and a low molecular weight component. The disclosed resins had a normal comonomer distribution in that the low molecular weight component had a larger amount of comonomer than did the high molecular weight component. We have now found that by adding more comonomer to the high molecular weight component of these resins, we can improve the ESCR properties. The polyethylene compositions provided by the present invention also have good organoleptic properties, balanced rheological and mechanical properties and are suitable for use in the manufacture of closures for bottles.